Combustion engines are currently favored as the means of propulsion for automobiles and many other motorized vehicles. The combustion engine's main attribute is its tremendous power to mass ratio (1 kW/kg). One drawback of these engines is their limited efficiency. Furthermore, gear boxes are required because optimum torque is delivered only in a narrow range of operation. The use of gear boxes increases mass and reduces efficiency. Also, there is a limited ability to divide the torque delivered to each wheel, thus, in order to supply each wheel with the requisite torque, each wheel, ideally, would have to be fitted with its own direct drive motor. In the case of combustion engines, this is not an attractive solution. Indeed, direct drive combustion engines are not feasible because of the limited range of angular velocities over which torque is effectively delivered.
Electric motors offer an attractive alternative to combustion engines due to their high electrical to mechanical efficiencies, and negligible emissions. On the other hand, however, they suffer from very low torque to mass ratios.
Shape memory allow actuators and conducting polymer actuators offer very high force-to-mass and power-to-mass ratios such as required for effective direct drive motors. Conducting polymers actuators, the subject of the present invention, are expected to produce power densities of the order of 100 kW/kg and are potentially much more efficient than shape memory alloy actuators.
Conducting polymers feature a conjugated carbon backbone. Some common conducting polymers are polyaniline, polypyrrole and polyacetylene. These materials are semi-conductors. However, upon oxidation or reduction of the polymer, conductivity is increased. The oxidation or reduction leads to a charge imbalance, which in turn results in a flow of ions into the material in order to balance charge. These ions or dopants enter the polymer from a surrounding, ionically conductive, medium, such as a gel, a solid electrolyte or a liquid electrolyte. If ions are already present in the polymer when it is oxidized or reduced, they may exit. The mass transfer of ions both in and out of the material leads to a contraction or expansion of the polymer. Typical volume changes are on the order of 10%, and linear dimensional changes are hence on the order of 3%. It is thought that oxidation and reduction may also lead to electrostatically and chemically induced changes in backbone dimension, which in turn lead to dimensional changes. Given the stiffness of the polymers, it is expected that stresses of up to 200 MPa, about 800 times greater than those generated by human muscle, will be possible. Currently the maximum stresses observed are on the order of 20 MPa. Bandwidths of 10 kHz are anticipated in conducting polymer actuators, although those observed to date are limited to 60 Hz.